A laser audio transmitter

Here’s a way of transmitting audio that makes it virtually impossible for someone else to listen in. Instead of sending radio waves bouncing all over creation, this uses the focused light of a laser to transmit audio. In the image above you can see the silver cylinder which houses the laser diode. It is focusing the beam on a light dependent resistor to the right which looks almost like a red LED due to the intensity of the light.

The simplicity of this circuit is fascinating. On the receiving end there is no more than the LDR, a 1.5V power source, and a headphone jack. The transmitter is not much more complicated than that. It includes an audio output transformer which boosts the resistance of the audio signal. This increase in resistance ensures that the laser diode modulates enough to affect the LDR on the receiving end. The transmitter uses a 3.3V supply. Check out the video after the break to hear the high quality of audio coming through the setup.

Even LEDs can pass radio frequencies. Incandescent lamps have a MUCH longer rise time, and even they can support audio transmission over a light beam. In the olden days before lasers, people used a flashlight bulb with series resistor in place of the speaker, or with a battery and carbon microphone. The receiver was a telescope with CDS photocell at the focus.

No “rise time” problems, despite what “somebody” may have claimed. A driver chip for the laser or LED may have rise time delays though, but not the LED.

boosts the resistance? as much as i love being pedantic, that’s a pretty terrible explanation. My understanding was that the transformer isolates the sound source and the secondary side voltage adds or lowers the voltage (because it is in series) from the power source which causes the laser to modulate.

Correct me if I’m wrong, but what you are really doing is you are amplifying the voltage coming out of the microphone. If the voltage is high enough, the battery won’t source any current. If the voltage is low, current starts to flow into the laser.

This also means that the laser is inverting your audio signal. In this case, it doesn’t matter if it is inverted or not. (It will eat up your battery life a bit)

You can verify this theory by putting an oscilloscope on the input and the output of the transformer without attaching the batteries. (Open circuit).

Thanks a lot. I promise to have an updated explanation about how this works soon. From my additional research so far, it seems I was on the right track, I just worded it very poorly. I mean, the number of turns on the coils does impact the resistance, but the real impact is the voltage change (as you stated).

It seems the goal is to create the change in impedance, which is directly affected by the resistance in the V, I, R relationship.

An LED across the speaker output of a stereo with the volume turned loud enough to make it blink. A photo-resistor, 9V battery and a PC speaker in series will pick up the audio if the light is focused on to the photo-resistor.

I did this back in the late 80’s with my home stereo. Being a curious kid, I stuck an LED into the speaker output of my stereo and turned the volume up until the LED started blinking.

Around the same time, I was playing with a photo-resistor, 9V battery and some old headphones wired in series, listening to the hum/buzz from lights, etc.

Then one day I had an epiphany – would the LED be blinking fast enough to transmit the sound? Sure enough it did!

In fact, even an incandescent light bulb was capable of transmitting some of the lower frequencies – enough that some songs could be identified (don’t remember if it broadcast speech).

Actually, Mim’s circuit in the late 70’s was based on 1960’s projects involving solar cells and lamps.

Naturally, before that, there were a variety of “photo phones”, and since modulation is a minor modification on the idea, the tech goes back to the bronze age when someone discovered that you could use a mirror to shine light over long distances. In Einstein’s early days, he worked on the conversion of light to electricity using silicon, aka the photoelectric effect.

The greeks used it for long range data transmission, at ungodly low data rates and terrible signal to noise ratios.

Technology is like sex – it’s always been done long before you tripped over it, but most people don’t talk about it.

C’mon HAD – “boosts the resistance” is a pretty diabolical explanation even by your standards. If the writer is unsure about the technical details, couldn’t someone else on the team help him get it right?

Anyway, anecdote time – we used a set up virtually identical to this receiver, pinching a laser from the school physics lab, to eavesdrop to what was being said in a building across the way – just refract the laser off the window glass. Awesome coolness.

A good source for small infraredLDs is dead optical mice.
Usually the diode doesen’t fail and it can be detached from the rest of the combined block using application of a hacksaw, Chipquik and a small screwdriver.
They typically use 1.9mA to generate a 0.7mW beam which is a great deal more efficient than a typical red LD.
(watch out, they can be Class 3 if focussed or run outside their specifications!)

The November 1970 issue (of Popular Electronics) also has an article by Forrest M. Mims and Henry E. Roberts titled “Assemble an LED Communicator – The Opticon.” A kit of parts could be ordered from MITS in Albuquerque, New Mexico. Popular Electronics paid $400 for the article.

Didn’t know most of the people visiting this Site were pompous assholes, now I know. “I did this years ago, so you’re lame.” shouldn’t you all be encouraging of someone getting into electronics? So disappointing. I for one think this is really cool, good work man. Old men will always be grumpy dicks I guess.